2008 Chevy Silverado 2500HD Allison Trans - HELP

We went to a memorial for our grandson who was serving at Ft Riley last week, I really don't need anything else to add to the stress.

Truck is a 2008 diesel with the Allison trans.  4x4.  About 104K miles.  Trans was serviced at about 80K miles.  All was fine when it was parked about 2 weeks ago.  Went to move it and it backed up fine.  Unloaded some stuff and now it won't move. Goes into gear, but motor just revs.  Put it in 4WD.  Nothing.  Put it in 2WD.  Nothing.  Drive and M1.  Nothing. 

Checked the fluid and it looks fine, but can't really tell the level since it is sitting on a slope, but fluid is up on the dipstick.

Frankly, feel like I'm going to puke.  Anyone have any ideas... please...

I had the same truck. 

It was notorious for electrical gremlins. I replaced several sub-harnesses. Two of them affected the trans controller and it’s ability to shift properly. 

I’m betting your Allison is fine, but you have electrical gremlins affecting it. 

Thanks SVreX. 

I have an update.  After sitting for a couple of hours, the truck moves, but only in 4WD Low range.  Refuses to go into 4WD Hi or 2WD.  Did you replace a sub-harness that might be involved in that?

At least it is out of the main driveway.

Was out looking in some other forums and a Chevy tech suggested to someone with the same issue that they start the truck in neutral, let it run for 30 seconds, and then move the selector to 2WD.

That worked.  Why?  Who the hell knows.

I berkeleying hate it when things magically cure themselves.  You never know what was wrong or if it going to happen again.

Thanks for your support, SVreX

At least you know how to fix a trans now!

Robbie said:

At least you know how to fix a trans now!

Yeah, just sprinkle it with pixie dust...

CJ said:

Thanks SVreX. 

I have an update.  After sitting for a couple of hours, the truck moves, but only in 4WD Low range.  Refuses to go into 4WD Hi or 2WD.  Did you replace a sub-harness that might be involved in that?

At least it is out of the main driveway.

Honestly, I don’t remember the exact details. I do know the trans controller was not talking to the ECU well. Dash indicators would say I was in 4WD low when I was actually in 2wd. Thing wouldn’t shift. Stuff like that. 

So yes, there may be som similarities. 

I also know that pixie dust didn’t fix it forever. It did come back, and I did have to repair it (more than once).  But it was electrical, not mechanical.

Hopefully you are all set now!  Best of luck!!